The present invention relates to a process to polymerize one or more monomers by means of one or more organic peroxides being dosed to the polymerization mixture at the reaction temperature.
Such a process is known from DE-OS-1 570 963. Herein it is taught to dose an initiator, optionally mixed with a solvent, to the composition being polymerized via a stream of water. The organic peroxides that are exemplified in this patent application are peroxydicarbonates and acetyl-cyclohexyl-sulfonyl-peroxide (ACSP). They are used at a temperature of 54xc2x0 C. The half-life of peroxy-dicarbonates at 54xc2x0 C. is known to vary from about 3.5 to 4.5 hours, depending on the type of peroxydicarbonate used.
This process of DE-OS-1 570 963 was found to solve a number of problems in the industry. However, the process still suffers from insufficient control of the heat peak of the polymerization reaction, a related less than optimal use of the reactor, a rather poor initiator efficiency, high residual peroxide levels in the resin produced, especially of the peroxydicarbonates, and/or the use of an undesirable initiator like ACSP which is known to lead, inter alia, to undesired fish eyes in the resin. A high residual peroxide concentration, especially of ACSP, is considered to correlate with a poor thermal stability of the resin containing it, which in turn is associated with a discolouration of the resin upon further processing. Accordingly, a different process not suffering from these drawbacks is desired.
Similarly, EP-A-0 096 365 discloses how a peroxide is added in three parts during the polymerization. Again, difficulties in controlling the heat that is subsequently generated are reported.
The current invention relates to a new process wherein these problems have largely been solved. More particularly, we have found that by selecting the proper organic peroxide and the proper dosing conditions, it is possible to obtain a polymerization reaction where the heat of polymerization is virtually constant over time, allowing optimum reactor space-time yield, very efficient peroxide usage, resulting in high polymer yields on the initiator, very low residual peroxide levels in the resin after polymerization, low fish-eye levels in the resin, and low reactor fouling. Accordingly, a resin with improved heat stability properties and low fish-eye levels was obtained, while the polymerization time could be shortened.
The new process is characterized in that essentially all of the organic peroxide used in the polymerization process has a half-life from 0.05 hour to 1.0 hour at the polymerization temperature. This allows for accurate control of the polymerization rate and related polymerization heat generation by controlling the peroxide dosing rate, while also resulting in a high yield of resin with low levels of residual peroxide and low fish-eyes.
The process according to the invention is pre-eminently suited to polymerize monomer mixtures comprising vinyl chloride monomer (VCM). Preferably, the process according to the invention involves the polymerization of monomer mixtures comprising at least 50% by weight (% w/w) of VCM, based on the weight of all monomer.
Comonomers that can be used are of the conventional type and include vinylidene chloride, vinyl acetate, ethylene, propylene, acrylonitrile, styrene, and (meth)acrylates. More preferably, at least 80% w/w of the monomer(s) being polymerized is made up of VCM, while in the most preferred process the monomer consists essentially of VCM. As is known in the art, the polymerization temperature of such processes to a large extent determines the molecular weight of the resulting resin.
In the process according to the invention, one or more peroxides may be used, as long as essentially all peroxides that are used fulfill the half-life requirement. It is noted that in JP-A-07082304 also a peroxide with a half-life within the range of 0.05-1.0 hour at polymerization temperature is dosed. However, according to this reference, another, more stable, peroxide is used from the start of the polymerization. This more stable peroxide does not fulfill the specified half-life requirement, and we have observed that the resulting resin contains an unacceptably high residue of this peroxide and, accordingly, will suffer from poor heat stability, which is typically observed in the form of discolouration during further processing of the resin.
Preferred examples of peroxides to be used in the process according to the invention are the following:
1,1,3,3-tetramethylbutylperoxy methoxy acetate, for polymerization reactions at 35-50xc2x0 C., preferably 40-45xc2x0 C.
diisobutanoylperoxide, bis(tert-butylperoxy) oxalate or 2,2-bis(2,2-dimethylpropanoylperoxy)4-methyl pentane, for polymerization reactions at 40-65xc2x0 C., preferably 45-60xc2x0 C.
xcex1-cumyl peroxyneodecanoate, 2-(2,2-dimethylpropanoylperoxy)-2-(2-ethylhexanoylperoxy)-4-methyl pentane or 2,4,4-trimethylpentyl-2-peroxyneodecanoate, at polymerization temperatures of 53-79xc2x0 C., preferably 60-75xc2x0 C., and
tert-amyl, tert-butyl peroxyneodecanoate or peroxydicarbonates, at polymerization temperatures of 58-87xc2x0 C., preferably 75-80xc2x0 C. Other peroxides may also be used. They can be classified into any of the above-mentioned categories on the basis of the 0.05 and 1.0 hour half-lives as determined by conventional thermal decomposition studies in monochlorobenzene, as well-known in the art (see for instance the brochure xe2x80x9cInitiators for high polymersxe2x80x9d with code 10737 obtainable from Akzo Nobel). As mentioned above, the process according to the invention requires that essentially all peroxide be used at a polymerization temperature at which the half-life of the peroxide is from 0.05 hour to 1.0 hour. More preferably, essentially all peroxide that is used has a half-life of 0.05 to 0.8 hour, even more preferably 0.08 to 0.5 hour, most preferably 0.08 to 0.35 hour, at the polymerization temperature.